Method for the desulphurisation of an oxidation catalyst arranged in the exhaust line of a diesel internal combustion engine

ABSTRACT

A process is proposed for desulfurization of an oxidation-type catalytic converter  8  mounted downstream in the exhaust gas stream of a diesel internal combustion engine  1  with the diesel internal combustion engine  1  being operated at a lambda value λ 1  of approximately 0.98 to 0.99 and an exhaust gas temperature T 1  of at least 350° C. As a result the oxidation capacity of the oxidation-type catalytic converter  8  that is reduced by sulfur is restored within a short period of time and at moderate exhaust gas temperatures so that diesel internal combustion engines may also be operated at least temporarily with sulfurous fuel.

[0001] This invention relates to a process for desulfurization of an oxidation-type catalytic converter mounted in the exhaust gas stream of a diesel internal combustion engine. Such oxidation-type catalytic converters are used in diesel internal combustion engines in order to permit low-temperature regeneration of a carbon filter mounted downstream in the exhaust gas stream by oxidation of nitrogen oxides.

[0002] It is known in connection with desulfurization of No_(x) storage-type catalytic converters that diesel internal combustion engines are to be operated at an air-fuel ratio lambda situated noticeably below 1 and at an exhaust gas temperature above 550°. Desulfurization of No_(x) storage-type catalytic converters, however, requires relatively long [periods] for restoration of their storage capacity.

[0003] Desulfurization of oxidation-type catalytic converters to restore their oxidation capacity, on the other hand, presents significantly greater problems. The attempt to remove sulfur in the form of sulfur bound on their surfaces from oxidation-type catalytic converters by high temperatures, high loads, and high speeds has yielded inadequate desulfurization. In addition, because of the high temperatures the danger exists of thermal damage to the oxidation-type catalytic converters in a procedure of this nature.

[0004] Consequently, it has thus far been necessary to rely on operation of a diesel internal combustion engine exclusively with low-sulfur or zero-sulfur fuel to make regeneration of a downstream carbon filter possible and thus prevent breakdown of the diesel internal combustion engine.

[0005] On the basis of this state of the art the object of this invention is to provide a process for desulfurization of an oxidation-type catalytic converter mounted downstream in the exhaust gas stream of the diesel internal combustion engine, a process which permits considerably improved desulfurization without entailing the danger of thermal damage to the oxidation-type catalytic converter.

[0006] This object is attained in that the diesel internal combustion engine is operated for desulfurization of the oxidation-type catalytic converter at a lambda value λ1 of around 0.98 to 0.99 and an exhaust gas temperature T1 of at least 350° C. By means of the exhaust gas produced in this manner, with its high content of carbon monoxide CO, hydrogen H₂, and hydrocarbon HC, the —SO₄ sulfates formed on the surface of the oxidation-type catalytic converter are spontaneously reduced to carbon dioxide SO₂, so that a reduced oxidation capacity of the oxidation-type catalytic converter is restored in a maximum of three to four minutes even at moderate exhaust gas temperatures and so ones suitable for practical application. A regeneration period made up of the sum of several brief “rich stages,” ones lasting seconds, for example, can be achieved. Because of the process of desulfurization claimed for the invention, it is possible to operate the diesel internal combustion engine at least temporarily with sulfurous fuel, this a necessity among other things in travel in countries not having zero-sulfur fuel. But even if the diesel internal combustion engine is operated with low-sulfur or zero-sulfur fuel the desulfurization process has a positive effect on the service life of the oxidation-type catalytic converter.

[0007] Preference is to be given to conduct of desulfurization of the oxidation-type catalytic converter under high load and in operation of the diesel internal combustion engine at a low speed, as for example during an acceleration process under full load and at an engine speed of 1250 rpm. The reason is that the lambda value required for the desulfurization process, a value slightly lower than 1, is to some extent already available, depending on the type of diesel internal combustion engine, so that only slight additional enrichment of the air-fuel mixture is in any event required.

[0008] It is advisable for the intake air to be throttled in order to achieve the lambda value λ1 or the exhaust gas temperature T1.

[0009] If this is not sufficient, speed-synchronized afterinjection may be carried out, in which case the afterinjection is to be effected a maximum of 20 degrees beyond the main injection, so that the diesel internal combustion engine may nevertheless be operated largely under low-carbon conditions.

[0010] In addition or as an alternative to the afterinjection, the boost pressure may also be lowered in order to reach the lambda value λ1 or the exhaust gas temperature T1.

[0011] With all these mixture-enhancement measures an effort should always be made to achieve low-carbon operation of the diesel internal combustion engine in order to avoid additional charging of the carbon filter, if possible.

[0012] It is advantageous for the desulfurization process to be conducted only when a regeneration requirement is set, the regeneration requirement being set as a function of the extent of sulfur fouling of the oxidation-type catalytic converter. As a result, the mixture enrichment measures are not carried out unnecessarily if the oxidization capacity of the oxidation-type catalytic converter is still fully acceptable.

[0013] If the extent of sulfur fouling of the oxidation-type catalytic converter is determined as a function of the hours of operation, fuel flow rate, and/or exhaust gas temperature, it being assumed that the sulfur content of the fuel is known, a reliable criterion is obtained for determining when a desulfurization process is due.

[0014] If in addition determination is made of the time during which the diesel internal combustion engine is operated at the lambda value λ1 and above the exhaust temperature T1, a criterion is derived for determining successful completion of a desulfurization process.

[0015] The process for desulfurization of an oxidation-type catalytic converter mounted in the exhaust gas stream of a diesel internal combustion engine is explained in detail in what follows with reference to the following FIGURE.

[0016] The FIGURE shows a diesel internal combustion engine 1 with a diagrammatic representation of an injection device 2. The intake air moves over an intake line 3 and a suction pipe 4 with built-in throttle 5 to the diesel internal combustion engine 1 and the exhaust gas moves by way of an exhaust gas line 6 to an exhaust gas turbocharger 7, an oxidation-type catalytic converter 8, and a carbon filter 9. In addition, a lambda sensor 10 and a temperature sensor 11 are mounted in the exhaust gas line 6 upstream from the oxidation-type catalytic converter 8.

[0017] To control a desulfurization process there is additionally provided a control device 12, which is connected to the injection device 2, the throttle 5, the exhaust gas turbocharger 7, and the two sensors 10 and 11.

[0018] In order to determine the degree of sulfurization (sulfur fouling) of the oxidation-type catalytic converter 8, the control device 12 continually acquires data representing, in addition to the lambda value, the hours of operation, the fuel throughput, and the exhaust gas temperature of the diesel internal combustion engine 1. When the upper limit is reached, the control device 12 issues a regeneration command. Lowering of the boost pressure is then carried out as a function of the operating point of the diesel internal combustion engine 1, throttling by means of the throttle 5, speed-synchronized afterinjection by means of the injection device 2, and/or lowering of the charge pressure by means of the exhaust gas turbocharger 7, in order to reach an air-fuel ratio λ1, of 0.98 to 0.99 and an exhaust gas temperature T1 of at least 350° C. required for desulfurization of the oxidation-type catalytic converter 8.

[0019] Once the lambda value λ1 and the exhaust gas temperature T1 respectively have been reached, the sulfates formed on the surface of the oxidation-type catalytic converter 8 are spontaneously reduced to sulfur dioxide, for example, in accordance with the formula

Al₂(SO₄)₃+3H₂→Al₂O₃+3SO₂+3H₂O.

[0020] As a result, the oxidation-type catalytic converter 8 can again effect adequate oxidation of the nitrogen oxides

NO+O₂→NO₂

[0021] and the oxidized nitrogen oxides may react in conjunction with carbon and oxygen in accordance with the formula

NO₂+C+1/2O₂→CO₂+NO.

[0022] Consequently, low-temperature regeneration of the carbon filter 9 mounted downstream from the oxidation-type catalytic converter 8 ultimately takes place.

[0023] The control device 12 additionally acquires data representing the time during which the lambda value λ1 is situated between 0.98 and 0.99 and the exhaust gas temperature above T1=305° C., that is, the time during which regeneration of the oxidation-type catalytic converter 8 is effected. After a specific regeneration period has been completed, such completion implying that the lower boundary value of the degree of sulfurization of the oxidation-type catalytic converter 8 has been reached, the regeneration command is withdrawn and the diesel internal combustion engine resumes normal operation. 

1. A process for desulfurization of an oxidation-type catalytic converter mounted in the exhaust gas stream of a diesel internal combustion engine, characterized in that the diesel internal combustion engine (1) is operated at a lambda value λ1 of approximately 0.98 to 0.99 and an exhaust gas temperature T1 of at least 350° C.
 2. The process as claimed in claim 1, wherein the desulfurization is effected under a high load and at a low speed of the diesel internal combustion engine (1).
 3. The process as claimed in claim 1 or 2, wherein throttling of the intake air is effected in order to reach the lambda value λ1 and exhaust gas temperature T1 respectively.
 4. The process as claimed in one of claims 1 to 3, wherein speed-synchronized afterinjection is effected in order to reach the lambda value λ1 and exhaust gas temperature T1 respectively.
 5. The process as claimed in claim 4, wherein the speed-synchronized afterinjection occurs a maximum of 20 degrees beyond the main injection.
 6. The process as claimed in one of claims 1 to 5, wherein lowering of the boost pressure is effected in order to reach the lambda value λ1 and exhaust gas temperature T1 respectively.
 7. The process as claimed in one of claims 1 to 6, wherein the desulfurization is effected when a regeneration command has been issued, the regeneration command being issued as a function of the degree of sulfurization of the oxidation-type catalytic converter (8).
 8. The process as claimed in one of claims 1 to 7, wherein the degree of sulfurization of the oxidation-type catalytic converter (8) is determined as a function of the hours of operation, the fuel throughput and/or the exhaust gas temperature of the diesel internal combustion engine (1).
 9. The process as claimed in one of claims 1 to 8, wherein the time is determined while the diesel internal combustion engine (1) is in operation at a lambda value λ1 and above an exhaust gas temperature T1, so that the regeneration command may be withdrawn again after a specific regeneration period has been completed. 